Transformer coils used in high-voltage and other applications are formed by winding a conductor and casting and curing a thermosetting resin composition around the conductor windings to form a resin body covering the coil. The resin body provides dielectric properties to the transformer coil assembly, as well as holding the conductor windings in place. The resin also provides protection and more uniform thermal properties to the coil assembly. Without some form of support structure for the coil assembly, the resin may develop cracks during casting or during use when the assembly is subjected to external conditions, such as high temperature, high humidity, moisture penetration and the like, or due to internal factors, such as heat generation or stress due to high current flow, electrical fault conditions, and the like.
The resin body is subjected to thermal forces from coil temperatures well above ambient during operation due to I2R losses in the conductors, from eddy currents, from hysteresis losses in the core, and from stray flux impinging the axial ends of the windings. Further, the resin body may be subject to vibratory forces during operation. The resin body should satisfactorily restrain, resist, and withstand all of these forces over long term operation.